### Summa Logicae

The Summa Logicae is a textbook on logic by William of Ockham. It was written around 1323.

Systematically, it resembles other works of medieval logic, organised under the basic headings of the Aristotelian Predicables, Categories, terms, propositions, and syllogisms. These headings, though often given in a different order, represent the basic arrangement of scholastic works on logic.

This work is important in that it contains the main account of Ockham's nominalism, a position related to the problem of universals.

## Book I. On Terms

1. Chapters 1–17 deal with terms: what they are, and how they are divide into categorematic, abstract and concrete, absolute and connotative, first intention, and second intention. Ockham also introduces the issue of universals here.
2. Chapters 18–25 deal with the five predicables] of Porphyry.
3. Chapters 26–62 deal with the Categories of Aristotle, known to the medieval philosophers as the Praedicamenta. The first chapters of this section concern definition and description, the notions of subject and predicate, the meaning of terms like whole, being and so on. The later chapters deal with the ten Categories themselves, as follows: Substance (42–43), Quantity (44–49), Relation (50–54), Quality (55–56), Action (57), Passion (58), Time (59), Place (60), Position (61), Habit (62).
4. Chapters 63–77 onwards deal with the theory of supposition.

## Book II. On Propositions

1. On categorical propositions (1–20)
2. On the conversion of propositions (21–9)
3. On hypothetical propositions (30–7)

## Book III. On Syllogisms

### Part I. On Syllogisms

1. On categorical syllogisms (1–19)
2. On modal syllogisms (20–30)
3. On mixed syllogisms (31–64)
4. On syllogisms containing exponible propositions

### Part III. On Consequences

• The first 37 chapters of Part II are a systematic exposition of Aristotle's Topics. In Part III, Ockham deals with the definition and division of consequences, and provides a treatment of Aristotle's Topical rules.[1] According to Ockham a consequence is a conditional proposition, composed of two categorical propositions by the terms 'if' and 'then'. For example 'if a man runs, then God exists' (Si homo currit, Deus est).[2] A consequence is 'true' when the antecedent implies the consequent. Ockham distinguishes between 'material' and 'formal' consequences, which are roughly equivalent to the modern material implication and logical implication respectively. Similar accounts are given by Jean Buridan and Albert of Saxony.
• Chapters 38 to 45 deal with the Theory of obligationes.

### Part VI. On Fallacies (in 18 chapters)

Part IV, in eighteen chapters, deals with the different species of fallacy enumerated by Aristotle in Sophistical Refutations (De sophisticis elenchis).